StarDate Podcast

One of the brightest stars of the summer sky is performing double duty here in the dead of winter — it appears in both early evening and early morning. And from far-northern latitudes, it never sets at all — it's in the sky every hour of the day, all year 'round. Deneb is at the tail of Cygnus, the swan. The brilliant star forms one point of the Summer Triangle, which soars high overhead during the summer months. Deneb remains visible for all or most of the year, though, because it's quite far north in the sky. It's about 45 degrees from the North Star, Polaris. From the northern hemisphere, Polaris is always at the same point in the sky, day or night. Its altitude depends on your latitude. From 30 degrees north, it stands 30 degrees above the northern horizon. And from 50 degrees north, it's 50 degrees above the horizon. As Earth turns on its axis, any star that's within that range of Polaris remains in view all night. From Seattle or Duluth, for example, that includes Deneb. At this time of year, the star passes just above the horizon during the night. For skywatchers south of that range, though, Deneb does disappear — for anywhere from a few minutes to hours. From most of the United States, it's in the northwest as night falls now and sets a few hours later. But it climbs into view again before dawn — this time in the northeast. So keep watch for Deneb — a summer star that's performing double duty on winter nights.  Script by Damond Benningfield Support McDonald Observatory

If you squished the planets of our solar system much closer to the Sun, you'd have something similar to the system known as Kepler 90. The smaller planets are closer to the star, while the giant planets are farther out. The star is bigger, brighter, and heavier than the Sun, and about half the Sun's age. Most important, it's the only star system besides ours with eight known planets. The planets that are close to the star are a little bigger than Earth, the ones in the middle are bigger still, and the two at the outer edge of the system are giants. There's one big difference between Kepler 90 and the solar system. The Sun's farthest major planet is Neptune — about 30 times farther from the Sun than Earth is. But at Kepler 90, the most-distant planet is only as far out as Earth. That means the planets are crowded together — smushed in close to their sun. When our solar system was young, its giant planets moved much closer to the Sun. That could have scattered any planets that were even closer. But the planets of Kepler 90 appear to be stable. They align in such a way that the gravity of each planet helps keep the others in place — a tightly packed family of planets. Kepler 90 is in Draco. Although the star is much too faint to see without a telescope, you can get an idea of its location. It's low in the northwest at nightfall, to the upper right of the bright star Vega. And it's in the northeast at dawn, to the left of Vega.  Script by Damond Benningfield Support McDonald Observatory

A beautiful spiral galaxy spins into view in the northeast this evening, near the tip of the Big Dipper's handle. It actually consists of two galaxies: a large one that's interacting with a smaller one, with a “bridge” of stars and gas between them. M51 is the first galaxy whose spiral nature was seen. It was revealed by the Leviathan of Parsonstown — the largest telescope in the world at the time. It entered service in 1847 — 175 years ago. The Leviathan was the invention of William Parsons, the Earl of Rosse, in Ireland. He owned and operated a large estate, but was also interested in astronomy. And he designed a telescope that far outclassed anything else of the time. Its main mirror spanned six feet — two feet wider than the second-largest. The metal mirror had to be taken out and polished every six months, so Parsons made two of them. The mirrors fit into a tube that was 58 feet long. It was suspended between two brick walls, and assistants used cables and pulleys to move it. Parsons did some of the observing himself, but he also hired a professional. Together, they studied the odd, fuzzy objects known as nebulae. Astronomers hadn't figured out what they were. The Leviathan revealed that some are clusters of stars. But more than a hundred were spirals. Decades later, astronomers showed that these objects are separate galaxies of stars — beautiful spirals first resolved with the Leviathan of Parsonstown.  Script by Damond Benningfield Support McDonald Observatory

The Moon is bashful — it never shows us its backside. The same hemisphere always faces Earth. There's a bit of a “wobble” that allows us to see around the edges a little. In all, though, two-fifths of the Moon is always hidden from view. That part of the Moon is known as the back side, the far side, or the dark side. People sometimes complain when it's called the dark side. They argue that, over a cycle of phases, the near side and the far side receive equal amounts of sunlight. As a result, the far side is no darker than the side we always see. And they're absolutely right. The “dark-side” tag doesn't refer to how much sunlight it gets, though. Instead, it refers to the fact that the far hemisphere is hidden from us. And until the Space Age, we had no idea what it looked like — it was completely unknown. It's a lot like dark energy. It might not be dark, and it might not even be energy. For now, though, it remains unknown — a “dark” mystery for scientists to solve. Today, of course, we know a lot about the dark side of the Moon. Orbiting spacecraft have mapped it in great detail. It has fewer dark volcanic plains than the near side, and lots more of the jumbled-up bright areas. Most of the near side is in good view tonight — lit up by the Sun. And the bright star Aldebaran is close by. They wheel high across the sky during the night and set in the wee hours of the morning. Tomorrow: Putting a leviathan to work.  Script by Damond Benningfield Support McDonald Observatory

Life on Earth is sustained by photosynthesis. Plants and some bacteria combine water, carbon dioxide, and sunlight to make sugars and oxygen, which they release into the air and water. Without that process, modern life wouldn't exist. Scientists have been wondering whether the most common stars in the galaxy might sustain photosynthesis on their planets. M dwarfs are the smallest and coolest of all stars. But they probably account for more than half the stars in the galaxy, which makes them of special interest in the search for life. In addition, M dwarfs live a long time, so there's plenty of time for life to develop. And because the stars are small and cool, any possible life-bearing planets would be close in, making them easier to find. The light produced by M dwarfs is different from sunlight. It's much redder, and there's a lot more infra-red. Scientists have considered whether those wavelengths could support photosynthesis. One study said that M dwarfs don't produce enough ultraviolet light. But another said the spectrum of an M dwarf should be just fine for life. And yet another one, released last year, agreed. That study subjected bacteria that live by photosynthesis to lighting conditions like those around M dwarfs. And the organisms did well. The issue is complicated by the fact that most M dwarfs produce big outbursts of X-rays. But it seems at least possible that microscopic life could get by on these little but common stars.  Script by Damond Benningfield Support McDonald Observatory

Astronomers have discovered thousands of planets in other star systems. So far, though, only a few can possibly be described as “Earth-like.” And two of those are in the same star system. Teegarden's Star is a close neighbor — just 12 light-years away. But it's so small and faint that it was discovered only a couple of decades ago. It's a fraction the size and mass of the Sun, and less than a thousandth as bright. Its two known planets are cataloged as objects “b” and “c” in the system. Both planets are just a few percent bigger and heavier than Earth. And both of them appear to lie in the “habitable zone” — the region where temperatures are just right for life. Since the star is so feeble, that zone is quite close — just a few million miles out. A study a couple of years ago said that both planets could have comfortable atmospheres and liquid water. But a study last year said otherwise. It found that gravitational interactions between the planets and the star could have caused problems. They could have turned b into something like Venus, with a hot, thick atmosphere. And c could have been turned into a world of volcanoes — making both planets unfit for life as we know it. Teegarden's Star is in Aries, the ram, which is high in the southern sky on January evenings. The star is much too faint to see without a telescope. But its location is easy to spot tonight — just above the Moon. More about extraterrestrial life tomorrow.  Script by Damond Benningfield Support McDonald Observatory

If you're a mighty Greek hunter who's tired from being out with the dogs all day, then it probably feels pretty good to put your feet up. And fortunately for the celestial hunter, the people who drew the early constellations provided a convenient footstool. Orion the hunter is well up in the east and southeast as night falls. Look for his three-star belt aiming almost straight up from the horizon. His foot is to the right of the belt — the blue-white star Rigel. A fainter star stands just above Rigel. It's known as Cursa — from a longer name that means “the footstool of the Central One” — the Central One being Orion himself. Yet the footstool doesn't belong to the constellation Orion at all. Instead, it's one of the brighter lights of Eridanus, the river. It's a long, faint ribbon of stars that meanders far to the right and lower right of Orion. Cursa is about 90 light-years away — much closer than Rigel. It's roughly twice as big and heavy as the Sun. And it's classified as a stellar giant. It's used up the original hydrogen fuel in its core, causing its outer layers to begin puffing up like a balloon. Over time, it'll ignite a new series of nuclear reactions, which will make it get much bigger than it is now. It'll also get much brighter, and it'll get cooler, which will make its surface shine yellow, then orange. All of those changes will make the footstool even more impressive than the foot it's there to support.  Script by Damond Benningfield Support McDonald Observatory

Two bright planets are pinching closer together in the early evening sky. They won't quite catch up to each other, but they'll get pretty close. Mercury and Saturn are quite low in the southwest at sunset. They quickly come into view as twilight begins to fade away. The best time to look for them is about 30 minutes to an hour after sunset. Mercury is the brighter of the two worlds — for now — with Saturn to its upper left. Both are easy to spot, but only if you have a clear horizon. Mercury is actually making one of its better evening appearances of the entire year. It's the closest planet to the Sun, so it never strays far from the Sun in our sky. At best, it's visible for a little while after sunset or before sunrise. Saturn, on the other hand, is the sixth planet from the Sun — far outside Earth's own orbit. So the giant planet crosses the entire night sky. Right now, it's just finishing up another crossing, so it's dropping toward the Sun, and soon will pass behind it. For now, look for these worlds in the southwest in early evening. They'll be closest together on Thursday, separated by less than four degrees — the width of two fingers held at arm's length. Mercury is fading with each passing day, though. As it drops away from Saturn, later in the week, it'll also drop below Saturn's brightness. It'll be lost in the twilight by about next weekend, with Saturn disappearing a few days later.  Script by Damond Benningfield Support McDonald Observatory

The planet Venus is changing things up this week. Tonight, it crosses the line between Earth and the Sun — a moment called inferior conjunction. As it does, it moves from the evening sky to the morning sky. Within a few days, it'll climb into view low in the east-southeast during the dawn twilight — as the brilliant “morning star.” Venus is at its closest now — just 25 million miles away. Yet the planet doesn't shine at its brightest, as you might expect it to. That's because it's a very thin crescent. The Sun is lighting up the hemisphere that's facing away from Earth right now — just like a new Moon. That means it's nighttime on the hemisphere of Venus that's facing us, with only a thin sliver in the sunlight. Over the next few months, Venus will move farther from the Earth-Sun line, so the Sun will shine on more of the planet's Earth-facing side. In other words, Venus will be getting fuller. That'll make it brighter. At the same time, though, Venus will be moving farther from us. That will make it fainter. When you combine those factors, Venus will shine at its brightest in mid-February. Regardless of how bright it looks on any given date, though, Venus always ranks as the second-brightest object in the night sky — only the Moon outshines it. Look for Venus beginning later this week, quite low in the late dawn twilight. It'll stay in the morning sky all the way through summer — the beautiful “morning star.”  Script by Damond Benningfield Support McDonald Observatory

A star system in the unicorn stays busy. An old, puffy star pours gas onto a small, dead companion. That forms a bright disk, high-speed jets, and powerful outbursts — with perhaps a much bigger outburst in the system's future. MWC 560 is in Monoceros, the unicorn, which follows bright Orion across the sky. The “dead” star is a white dwarf — the hot, dense core of a once normal star. The other star is a red giant — a star at the end of its life. It's puffed up to giant proportions, so its grip on the gas at its surface isn't very strong. That allows the gravity of the white dwarf to pull some of its gas away. The gas forms a wide, thin disk around the white dwarf. Some of the gas falls onto the white dwarf, making the star heavier. But some of it shoots back out into space in skinny jets. They reach speeds of millions of miles per hour. Earth lines up directly in the path of one of the jets. MWC 560 has grown much brighter several times in the last few decades. The most recent outburst started in 2016, and it took a while to fade. The companion probably was dumping more gas onto the white dwarf. As a result, the jet that faces Earth got thicker and faster, carrying a lot more material away from the white dwarf. It's possible that the white dwarf could become a nova later in this century. That would blow its outer layers into space, making it shine thousands of times brighter — a powerful eruption from a busy star system.  Script by Damond Benningfield Support McDonald Observatory

When two stars come together, the result can be spectacular. Twenty years ago today, for example, astronomers observed a brilliant eruption in Monoceros, the unicorn. It probably was triggered by the merger of two stars. At its peak, the outburst was a million times brighter than the Sun. The system is known as V838 Monocerotis. It's about 20,000 light-years from Earth. It's on the rim of the Milky Way Galaxy, in its outermost spiral arm. And it appears to belong to a small cluster of bright, heavy stars. The system became famous a couple of years after the outburst thanks to Hubble Space Telescope. Its pictures showed a brilliant red star surrounded by rings that looked like puffs of cotton candy. The rings weren't related to the stars, though. Instead, they were clouds of dust around the stars that were being lit up by the eruption. V838 Mon originally consisted of at least three stars. Two of them were especially big — roughly eight times the mass of the Sun. The third star was only about a third the Sun's mass. And on January 6th of 2002, as viewed from Earth, the small star appears to have plowed into one of the bigger ones. That triggered the eruption — a spectacular lightshow from a pair of merging stars. Monoceros climbs into view in the east and southeast by about 8 p.m., to the lower left of Orion. The unicorn is quite faint, though, so you need dark skies to see it. More about the unicorn tomorrow.  Script by Damond Benningfield Support McDonald Observatory

The American space program is gearing up to send astronauts back to the Moon. A test flight of their new capsule and booster is scheduled for later this year. A series of robotic missions will begin this year as well. Many of those missions will look for frozen water and other resources to sustain human expeditions. A half-century ago, NASA was winding down its first wave of lunar exploration. The final Apollo missions were scheduled for launch in 1972, and the space agency was planning its next step. And 50 years today, it got the okay to go ahead. President Richard Nixon approved the space shuttle — a reusable rocket ship to carry astronauts to Earth orbit quickly, reliably, and often. It took almost a decade to get the shuttle ready to fly, though. And it never performed the way NASA said it would. It cost more, flew less, and proved to be tricky to operate. Two of the shuttles were destroyed in flight, killing 14 astronauts. Yet the shuttle did ferry satellites to space, including Hubble Space Telescope and a mission to Venus. Astronauts conducted scientific research, and assembled the International Space Station. The shuttle was retired a decade ago — 40 years after it was given a “go” to proceed. The destination for the next generation of spaceships — the Moon — has a brilliant companion tonight: the planet Jupiter. It's close to the upper right of the Moon as night falls. They drop from view about 8:30 or 9.  Script by Damond Benningfield Support McDonald Observatory

Saturn's rings are beautiful to look at. For scientists, they're also valuable instruments — they provide a glimpse deep inside the giant planet. The main rings span about three-quarters of the distance from Earth to the Moon. They consist mainly of ice — from tiny grains to big boulders — with a smattering of dust and rock. The Cassini spacecraft studied the rings as it orbited Saturn for more than a decade. It found that the rings are constantly changing. And some of those changes are caused by what's happening inside Saturn. Motions deep within the planet cause subtle changes in Saturn's gravitational and magnetic fields. A few years ago, scientists used those changes to measure the length of Saturn's day. That had been hard to pin down because Saturn doesn't have a solid surface. But Cassini revealed waves rippling through the rings. The waves told scientists that a day on Saturn lasts a little more than 10 and a half hours. And this year, scientists used a wave pattern seen in one of the rings to suggest that Saturn's core is mushy. It contains a lot of rock and metal, but it's mixed with hydrogen and helium, which make up most of the planet. That would make the core gooey and poorly defined, like a ball of melting cheese — a discovery made possible by Saturn's rings. And Saturn is easy to find early this evening. It looks like a bright star quite close to the Moon. Its rings are visible through just about any telescope.  Script by Damond Benningfield Support McDonald Observatory

Earth is huddling close to the Sun for the next couple of days — it's closest for the whole year, in fact. It's at a point in its orbit called perihelion, and it happens every January. There are a couple of ways to measure the separation between Earth and the Sun. There's the way astronomers measure it, which is from the center of Earth to the center of the Sun — 91.4 million miles at perihelion. Or you can measure it from the surfaces of the two bodies, which puts the Sun almost half a million miles closer. The distance between Earth and the Sun changes because Earth's orbit is an ellipse — like a slightly flattened circle. Over the course of an orbit, the distance varies by about three million miles. When the Sun is closest, as it is now, we receive about seven percent more total solar energy than when it's farthest, in early July. But that has nothing to do with the seasons. They're controlled by Earth's tilt on its axis. Right now, the north pole is tilted away from the Sun, so it's winter in the northern hemisphere, and summer in the southern hemisphere. The changing distance does have one important impact on the seasons. When Earth is close to the Sun, it moves faster in its orbit than when it's farther away. So that makes the length of the seasons uneven. Here in the northern hemisphere, winter is only about 89 days long, compared to 93 days of summer — extra sunlight thanks to our lopsided path around the Sun.  Script by Damond Benningfield Support McDonald Observatory

The stars are eternal — by human standards, anyway. But the constellations are not. That's because the constellations were drawn by people. And what one person draws, another can erase. Consider Quadrans Muralis. It's the namesake for the year's first meteor shower, the Quadrantids, because the meteors all appear to “rain” into Earth's atmosphere from that direction. But you won't find Quadrans Muralis on any modern starchart. French astronomer Joseph Lalande drew the constellation in 1795. He named it for the wall quadrant — an astronomical instrument that was used to plot the positions of stars. He carved the constellation out of a mostly barren patch of sky that's bounded by Boötes, Draco, and Hercules. Although several of Lalande's constellations caught on, this one didn't. Yet the meteor shower still bears the name of the extinct constellation. The shower itself should be at its best tomorrow night, with a peak rate of dozens of meteors per hour. If you trace the paths of the meteors across the sky, they all appear to originate in Hercules. But they can streak across any part of the sky, so you don't need to look toward Hercules to see them. The Moon is new today, so it's completely out of view — it won't dampen the fireworks. To give the Quadrantids a try, find a safe, dark viewing spot, away from city lights. Then bundle up and enjoy the show — a reminder of the changing landscape of the night sky.  Script by Damond Benningfield Support McDonald Observatory

Four of the five planets that are easily visible to the unaided eye line up in the southwest as night falls the next few evenings. But one of them won't stick around for long. A couple of weeks from now, it will have pulled into view in the morning sky. That one is Venus, which has reigned as the “evening star” for months. It's quite low as the Sun sets, so you need a clear horizon to spot it — any trees or buildings will block it out. But it's quite bright, so it's not hard to find. Venus is about to cross between Earth and the Sun. As it does so, it'll move from the sky to the morning sky. It'll become visible there in a couple of weeks, and be an easy target by the end of the month. The planet Mercury stands close to the upper left of Venus. It's moving higher into the evening sky, so it'll be a little easier to see over the next few days. It will vanish in the twilight in a couple of weeks. Mercury is climbing toward Saturn, which is to the upper left. They'll draw quite close next week, but they won't quite catch up to each other. Saturn will quickly follow Mercury into the twilight. The final planet is Jupiter, the giant of the solar system. It's to the upper left of Saturn, and shines brilliantly — second only to Venus. Jupiter will linger in the evening twilight for a while. It'll drop from view in mid to late February — then return to view in the morning sky in April.  Script by Damond Benningfield Support McDonald Observatory

The sky offers up some beautiful options for seeing the old year out and the new year in tonight. Shortly after the year's final sunset, look toward the southwest for a lineup of four bright planets — an array that includes the second- and third-brightest objects in the night sky. The brightest member of the quartet is Venus, the “evening star.” It's quite low in the sky, so you need a clear horizon to spot it — any clutter will block it from view. If you do see it, look close to the left for the fainter planet Mercury. Golden Saturn stands to their upper left. And brilliant Jupiter — the solar system's largest planet — is about the same distance to the upper left of Saturn. The planets are all gone from view by midnight. But some especially bright stars take their place. Orion the hunter is high in the south. Look for a rectangle of four bright stars with a short line of three stars at its center — Orion's Belt. Follow the belt to the lower left and you'll come to the Dog Star, Sirius, the brightest star in the night sky. And if you have dark skies, look for the Milky Way arcing high overhead. If you plan to watch the dawn of the new year, look low in the southeast for a pair of orange “eyes.” The one on the left is Mars, while the other is Antares, the star that marks the heart of the scorpion. And as twilight brightens, a fingernail crescent Moon rises below them — a tough target for the dawn of the new year. Script by Damond Benningfield Support McDonald Observatory

The Moon looks down on a couple of ancient “rivals” before and during dawn tomorrow — the planet Mars and the star Antares. They're quite low in the southeast at first light. Mars is close to the lower left of the Moon, with Antares about the same distance to the lower right. Mars is one of the reddest objects in the night sky, which has always drawn attention. Long ago, that color reminded skywatchers around the Mediterranean of blood, so they named the planet for the god of war. In Rome, the god was called Mars. But in Greece, he was Ares. And that's where the star Antares got its name. The star is bright and orange, like Mars, so it was named “Ant-Ares” — the rival of Ares. The comparison was easy to make because Mars passes by the star every couple of years or so. When they're close together, they really can look a lot alike — but not always. Antares always looks the same — it doesn't change. But as Mars orbits the Sun, it gets a lot brighter or fainter depending on its distance from Earth. At times, it's a good bit fainter than Antares. At others, it's many times brighter. Mars is just emerging from behind the Sun now, so it's a long way away, which makes it fainter than Antares. But by next December, Mars will line up opposite the Sun, so it'll be closest to us for the year. It will shine about 20 times brighter than it is now — truly outshining its colorful rival. Tomorrow: seeing in the new year with the stars. Script by Damond Benningfield Support McDonald Observatory

The Sun's closest planets are crossing paths in the southwest in early evening. There's a limited viewing window because they set by the time twilight fades away. The brighter planet is Venus. It's the “evening star,” quite low in the sky at sunset. It's so bright that you might mistake it for an airplane with its landing lights turned on. But it's so low that you need a clear horizon to pick it out — any trees or buildings will block it from view. Mercury stands a little to the lower left of Venus this evening, but it's moving upward. It'll be roughly even with Venus tomorrow evening, then will move up and away from it. It'll stand a little higher in the sky at sunset for several days. Mercury is the closest planet to the Sun, while Venus is the next one out. Yet Venus is much hotter. That's because it has a thick atmosphere, made mainly of carbon dioxide. As we know from our changing climate here on Earth, C-O-2 is a potent greenhouse gas — it traps heat. It's warmed the surface of Venus to more than 850 degrees Fahrenheit. By comparison, peak temperatures on Mercury — which has no atmosphere to speak of — are about a hundred degrees lower. And most of the time, across most of the planet, it's not nearly as hot. It can reach hundreds of degrees below zero on the nightside and at the poles — frigid conditions close to the Sun. Keep an eye on these close, hot worlds as they cross paths in the early evening sky. Script by Damond Benningfield Support McDonald Observatory

Earth is in a danger zone. Asteroids could smack into our planet, causing massive destruction. To protect ourselves, we need to know about them well in advance. So NASA and others are scanning the skies to locate these potential killers. Some of those efforts are based in space. NeoWise, for example, has discovered hundreds of asteroids that come close to Earth's orbit — including more than 60 that are listed as especially threatening. NeoWise was launched in 2009 to study the infrared universe. It shut down when it used up its coolant. But it was revived a couple of years later to look for asteroids, and it's been working ever since. And this summer, it got a two-year extension. NASA's already planning its successor. Near-Earth Object Surveyor would also scan the sky in the infrared. Asteroids are dark, but they produce a lot of infrared energy, so that's the best way to find and track them. The new craft is targeted for launch in 2026. Scientists also are working on a plan to send a craft to tag along with Apophis, an asteroid that will pass less than 20,000 miles from Earth in 2029. The craft spent a couple of years orbiting another asteroid, and is bringing samples to Earth. After dropping them off, in 2023, it could take aim at Apophis. It could reveal details about the asteroid and how its orbit is affected by Earth and the Sun — critical details that could help track all dangerous asteroids. Script by Damond Benningfield Support McDonald Observatory

What you know and what you can prove aren't always the same. More than four centuries ago, for example, many astronomers knew that Earth wasn't the center of the universe — that Earth and the “planets” revolved around the Sun. But proof was elusive. Until Johannes Kepler came along. Kepler was born 450 years ago today, to a family of modest means in present-day Germany. As a student, he studied theology, astrology, and science. All three of them shaped his view of the cosmos — as a divine creation that could reveal its secrets while guiding the future. In 1600, Kepler became an assistant to Tycho Brahe, perhaps the best naked-eye skywatcher ever. When Tycho died the following year, Kepler inherited Tycho's job — and his decades of meticulous observations of the night sky. It took him several years to work it all out, but Kepler used those records to formulate new ideas about the universe. Those ideas are known today as Kepler's laws of planetary motion. Among other things, they describe how Earth and the other planets orbit the Sun, and how the Moon orbits Earth. Later in the century, Isaac Newton used Kepler's laws to form his own laws about the universe — especially the laws of gravity. Kepler contributed to science in many other ways. He explained how vision works in the human eye and in the telescope, for example. But his greatest work remains his laws of planetary motion — laws that proved what others only “knew.” Script by Damond Benningfield Support McDonald Observatory

Many stars have great names. But one of the best has to be Ra's al-Ghul — better known as Algol, “the head of the ghoul” or the demon star. It represents Medusa, a monster from Greek mythology. Its head was covered in snakes, and one glance at her instantly turned the viewer to stone. The name probably was bestowed because Algol does something that skywatchers in ancient times found a bit scary: It periodically gets a lot fainter. The heavens were thought to be eternal and unchanging, so such a star was considered a big problem. The cause of its behavior isn't scary at all, though. Algol includes two stars that are locked in a tight orbit around each other — they're just a few million miles apart. One of the stars is big and bright — almost 200 times brighter than the Sun. The other star is much smaller and fainter. Every two days, 20 hours, and 49 minutes, the fainter star passes in front of the brighter one. That causes the system to drop to just a third of its normal brightness. An eclipse lasts about 10 hours, with about four hours at minimum brightness. After that, the fainter star moves out of the way, and Algol roars back to full brilliance. Algol's next eclipse will take place tomorrow night. The star is high in the east-northeast as night falls, in Perseus the hero — the character who killed Medusa. If you can find Algol, check it out tonight, then again late tomorrow night — to see the action of a “ghoulish” star. Script by Damond Benningfield Support McDonald Observatory

In 1969, the Pentagon found itself with a stack of large telescope mirrors that it couldn't use. It had planned to launch them on Manned Orbiting Laboratory, a small space station. Astronauts would use the six-foot mirrors to spy on the Soviet Union and its allies. But improvements in robotic spy technology made the lab obsolete. So six of the mirrors eventually were given to the University of Arizona. It combined them to make a single telescope — the third-largest in the world at the time. The Multi-Mirror Telescope operated from 1979 to 1998, when the mirrors were replaced by a single piece of glass. Defense agencies have given several other big gifts to astronomy as well. In 1976, for example, the Air Force gave a whole observatory. It was built on a mountain peak in New Mexico after World War II to study the Sun. The Pentagon wanted to know how the Sun might affect radio communication, radar, and other technologies, so it built telescopes to find out. The observatory is still in operation. And NASA is preparing the Nancy Grace Roman Space Telescope for launch later this decade. It uses one of two spy satellites the National Reconnaissance Office gave to NASA a decade ago. Its primary mirror is the same size as the one in Hubble Space Telescope. But Roman's field of view is a hundred times wider. The telescope will study dark energy, exoplanets, and other leading topics — thanks to an astronomical gift. Script by Damond Benningfield Support McDonald Observatory

The night sky offers a few gifts to everyone this week, including a string of planets, a beautiful Moon, and the pre-eminent constellation of winter. Shortly after sunset tonight, as the colors of twilight get bolder, look toward the southwest for three planets. They're strung out like bulbs on a strand of Christmas lights. The brightest of the three is Venus, the brilliant “evening star.” Fainter Saturn stands to the upper left of Venus, by about one-and-a-half times the width of your fist held at arm's length. And Jupiter — second only to Venus in brightness — is about the same distance to the upper left of Saturn. Venus drops from view as the sky gets fully dark. By then, Orion is in view in the east. It's perhaps the most beautiful of all the constellations, and the easiest to find. Look for its three-star “belt” extending up from the horizon. It's flanked by bright orange Betelgeuse to the left, and blue-white Rigel to the right. A couple of hours later, the brightest true star in the night sky climbs into view below the belt: Sirius, the Dog Star. Finally, the Moon heaves into view by 11 or 11:30. Regulus, the bright heart of the lion, is far to the upper right of the Moon. Almost everything will look the same tomorrow night, with the exception of the Moon. It'll rise later and stand farther below Regulus, and it won't be quite as full or bright. So enjoy the view of the night sky — an extra present for this holiday week. Script by Damond Benningfield Support McDonald Observatory

It's easy to track the phases of the Moon. They're on just about every printed calendar, plus countless apps and web sites — including ours. That allows you to find the phase on any date far into the future. People have been devising ways to track the Moon for thousands of years. One of the earliest could be a bone found in Africa. Columns of notches could represent the phases of the Moon. The Ishango bone was found in present-day Congo, in the remains of a village that was buried by a volcanic eruption. The village was inhabited about 20,000 years ago. The bone is about four inches long. It's the leg bone of a baboon, topped by a crystal of quartz. The bone contains three columns of notches. The columns are divided into groups, with each group containing a different number of notches. The notches could be mathematical. They could represent a counting system, or even serve as a calculator, helping the owner do simple addition or multiplication. On the other hand, the notches could represent the phases of the Moon. The notches in each group are a different length, so they could represent the waxing and waning of the Moon, covering about six months of phases — an early “calendar” for tracking the Moon. Tonight, the Moon is about three-quarters full, so it's nice and bright. And it has a bright companion: Regulus, the heart of the lion. They climb into view in late evening, and soar high overhead later on. Script by Damond Benningfield Support McDonald Observatory

Many ancient cultures thought of the stars as fixed and unchanging. Any change they did see was a bad sign. A star in Perseus that gradually fades and brightens, for example, was called the demon star. Goodness only knows what they would have called Betelgeuse, the orange shoulder of Orion. It's normally the 10th-brightest star in the night sky. In late 2019 and early 2020, though, it dropped out of the top 20. It faded to only about a third of its normal brightness, although it eventually recovered. Betelgeuse is a supergiant. It's much bigger, heavier, and brighter than the Sun. The exact numbers are uncertain, mainly because its distance is uncertain. A recent study put it at about 550 light-years — closer than earlier studies. Betelgeuse is only about nine million years old, yet it's near the end of its life. It “burns” through its nuclear fuel at a furious rate. When it's through, it'll explode as a supernova — a blast that'll be visible in the daytime for weeks. Astronomers expect that to happen in the next hundred thousand years or so. So when Betelgeuse faded, there was some speculation that it was a prelude to the final blast. Most studies, though, say the star faded because it blew a giant blob of gas into space. The blob cooled and darkened, making Betelgeuse fade away. Still, we know it's only a matter of time until the star's brilliant end — an event that would have been really scary for long-ago skywatchers. Script by Damond Benningfield Support McDonald Observatory

Mars is beginning to creep into the dawn sky. It's quite low in the southeast during twilight, so it's tough to spot. The planet is rising a little earlier each day, though, so it'll be a much easier target by the end of the month. The Perseverance rover has been trundling across the Red Planet since February. It landed in Jezero Crater — a big hole in the ground that once was filled with water. Perseverance's early work has shown that the water probably stuck around for a while — long enough to make Jezero an inviting home for life. All that water disappeared billions of years ago. So did most of the rest of the water on Mars, which was warm and wet in the distant past. Scientists have been trying to figure out where it all went. A lot of it escaped into space. Mars's gravity is weak, so water molecules are easily carried into the upper atmosphere. There, they can be split apart by sunlight, and the individual atoms blown away by the solar wind. And a recent study found that Mars loses a lot more water when it's closest to the Sun, and when it has giant dust storms, which heat the air and carry water high into the sky. But the rate at which Mars loses water to space isn't nearly enough to account for everything it's lost. Other studies have concluded that a lot of it was chemically bound with minerals on the surface. And still more probably trickled deep below the surface — where it's awaiting discovery by future explorers. Script by Damond Benningfield Support McDonald Observatory

If you stand outside the next couple of days at local noon — the time the Sun appears highest in the sky — and face away from the Sun, you might notice that your shadow is especially long. In fact, it's the longest noontime shadow of the entire year. That's because tomorrow marks the winter solstice — at 9:59 a.m. Central Standard Time. It's the shortest day of the year in the northern hemisphere, and it marks the beginning of winter. The Sun scoots lowest across the sky on the solstice, which is why you cast an especially long shadow. The solstice is set up by Earth's tilt on its axis. At this time of year, the north pole tilts away from the Sun. That brings those of us north of the equator especially short days. And the farther north you go, the shorter the days. Miami, for example, will see about 10 and a half hours of sunshine. For Dallas and Los Angeles, which are a little farther north, it's about a half hour less. For Seattle, it's just seven and a half hours. And for Anchorage, it's a meager five and a half hours of sunshine. The numbers stay about the same for several days. That's because the Sun appears to “stand still” now — it rises and sets at the same point along the horizon. In fact, “solstice” means “the Sun stands still.” Before long, though, it'll begin to move more quickly. Its rising and setting points will shift, and it'll pass a little higher across the sky — bringing longer days, but shorter shadows. Script by Damond Benningfield Support McDonald Observatory

Many people celebrate their birthdays under the stars. But when Ambrose Swasey turned 89, he celebrated under a machine his company was building to study the stars — the first telescope for McDonald Observatory. Swasey was born 175 years ago today, in a small town in New Hampshire. He quickly showed a knack for designing and building things. As a teenager, he went to work in a machine shop, where he met Worcester Warner. They formed their own company, Warner & Swasey, which they soon moved to Cleveland. The company mainly built machine tools. But both founders were interested in astronomy, so they started building telescopes and their protective domes as well — work that made the company well known. Warner and Swasey built a big new telescope for Lick Observatory in California, and a bigger one for the University of Chicago's Yerkes Observatory — still the biggest telescope of its type in the world. In 1923, Chicago astronomer Otto Struve named an asteroid discovered with the Yerkes telescope in Swasey's honor. Later, Struve became the Yerkes director. And when Chicago signed a deal with the University of Texas to operate its new McDonald Observatory, Struve became its director as well. He hired Warner & Swasey to build it, including its first big telescope — the second-largest in the world. The telescope has moved down the rankings since then. But it's still in service — one of the legacies of Ambrose Swasey. Script by Damond Benningfield   Support McDonald Observatory

Romantics and werewolves take note: There's a lot of moonlight the next few nights — more than at any other time of the year. The middle of the country, for example, will see about 14 and a half hours of moonlight the next couple of nights, after the day brings just nine and a half hours of sunlight. That extra lunar glow is the result of the Moon's phase and the time of year. The Moon will be full tonight. It's known as the Moon Before Yule. It lines up opposite the Sun, so it rises around sunset and sets around sunrise. And we're just a few days away from the winter solstice — the shortest day of the year in the northern hemisphere. The shortest days are accompanied by the longest nights. So December's full Moon is known as the Long-Night Moon. Not only does the Moon remain in view for a long time, it also climbs highest across the sky for the year. That's because the Moon lies near the ecliptic, which is the Sun's path across the sky. At this time of year, the ecliptic arcs low across the sky during the day because Earth's north pole is tipped away from the Sun. But the full Moon is on the opposite side of Earth, so the north pole tips toward it. As a result, the ecliptic climbs high across the northern sky — and so does the full Moon. The exact moment of the full Moon, by the way, is 10:35 p.m. Central Standard Time — the time of the Long-Night Moon. Tomorrow: a special line of business. Script by Damond Benningfield Support McDonald Observatory

The Sun will change addresses tonight. It'll move from Ophiuchus to the next constellation over, Sagittarius. That's where it'll reside at the winter solstice, next week. Sagittarius hasn't always been the Sun's home for the solstice, though. A couple of thousand years ago, the Sun was in Capricornus. And a century and a half from now, it'll move into Ophiuchus. The change is caused by precession — a slow “wobble” in Earth's axis. It causes the Sun to slide through the background of stars over the millennia. It also causes the north pole to trace a circle on the sky. It takes almost 26,000 years to make one circle. And during that time, Earth has different “north stars. An artist used precession to honor Hoover Dam. Oskar Hansen designed Monument Plaza. It includes a 14-story flagpole, two 30-foot statues, and a terrazzo floor that indicates the date of the dedication — September 30th, 1935 — with the help of the stars. Today, the North Star is Polaris. A line from the flagpole through a wide ring around it points to the position of Polaris at a precise time on the night of the dedication. Another line points to Thuban, the pole star when Egypt built the Great Pyramid. In fact, that was one of Hansen's inspirations: Architects used Thuban to align the pyramid — another great work of human history. And yet another line points to the bright star Vega. It will serve as the pole star in 12,000 years — thanks to precession. Script by Damond Benningfield Support McDonald Observatory

Every star is losing mass. Some are just a lot more serious about it. An example is Aldebaran, the eye of the bull. The star is below the Moon as night falls, and to the left of the Moon as they set, before dawn. Aldebaran has passed the end of its normal lifetime, so it's puffed up to giant proportions — it's about 45 times the diameter of the Sun. At that size, the gas at its surface is barely held in place by the star's gravity. In fact, a lot of it isn't held in place — it's blown into space as a high-speed wind. The wind carries away enough gas to make a planet as massive as Earth in about 300,000 years. That's about 500 times the rate at which the Sun blows gas into space. Much of the gas condenses to form grains of dust. They're creating a hazy cloud around Aldebaran. When the star dies, it'll expel all of its outer layers of gas into space. They'll be moving much faster than the present-day wind. As they ram into the older gas and dust, they'll create shock waves and sculpt “fingers” of dense material. The whole mixture of gas and dust will create a colorful bubble that will shine for tens of thousands of years. That's also in the future of the Sun. In about five billion years, it will puff up like Aldebaran has. The solar wind will get a lot thicker, surrounding the Sun with its own cocoon of gas and dust. Finally, the Sun will lose its outer layers and create its own bubble — the final outpouring of a star.  Script by Damond Benningfield Support McDonald Observatory

Tycho Brahe, who was born 475 years ago this week, in Denmark, was one of the most accomplished astronomers in history — and one of the most colorful. On the professional side, he was a great night-sky observer. Working before the invention of the telescope, he showed that a brilliant “new” star was far beyond Earth, for example — demonstrating that the heavens weren't unchanging, as most astronomers of the time believed. On the personal side, Tycho was larger than life. He was a descendant of several noble families, and was stolen away by his uncle when he was only two. In college, after a drunken argument over who was the best mathematician, he fought a sword duel with a cousin. He lost part of his nose, and wore a false nose — made of brass — the rest of his life. In 1572, he became royal astronomer to the king of Denmark. The king gave him an island, where Tycho built a major observatory and teaching center. But he quarreled with the locals, who objected to his heavy demands. After the king died, Tycho didn't get along well with the new king, so he left his island. He became imperial astronomer to the Holy Roman Emperor Rudolph the Second, and set up a new observatory near Prague. During a royal banquet, Tycho refused to leave the table to relieve himself. That caused a serious bladder problem. Tycho died from that problem days later — one final episode in the life of one of astronomy's most colorful characters.  Script by Damond Benningfield   Support McDonald Observatory

Tycho Brahe was having a hard time deciding what to do with his life. He'd studied astronomy, but he was also interested in alchemy — changing one element into another — as well as other fields. It took an exploding star to make up his mind. As he walked out of his laboratory one night in 1572, Tycho saw a brilliant new star in the sky — brighter than the planet Venus. He watched it for months. His work showed that the star was far from Earth. And that demonstrated that the heavens could change — an idea that went against the science of the day. That made him famous, and convinced him to concentrate on astronomy. Tycho was born 475 years ago today, in Denmark. After his writings about the “new star” — known today as Tycho's Supernova — he became royal astronomer to the king of Denmark. He built big observatories on an island. The telescope hadn't been invented yet, so he used instruments that helped him plot the stars with his eyes alone. And he was good at it — perhaps the best naked-eye observer in history. In addition to his work with stars, for example, Tycho showed that comets are far beyond Earth — not odd phenomena in the atmosphere, as most scientists thought. Tycho's observatory also was a center of learning, with dozens of students studying science and other fields. It remained busy until Tycho had a falling out with the new king. Yet his legacy was secure — as one of the great astronomers in history. More tomorrow.  Script by Damond Benningfield Support McDonald Observatory

A young star in Orion presents a mystery. More than a decade ago, it twice faded by a good bit, for several weeks each time. Astronomers had a possible explanation. But the star hasn't faded that way again, so they need a new explanation. PDS 110 is about a thousand light-years away. It stands close to Orion's Belt, which is in good view in the east by about 8 p.m. And PDS 110 might be related to the stars of the belt — they probably all formed from the same cloud of gas and dust. PDS 110 is only about 10 million years old. It may not even be a full-fledged star yet — it's just entering the prime of life. In 2008 and 2011, it faded by almost a third, for a few weeks each time. It was suggested that the star had a companion — a giant planet or a brown dwarf — that was encircled by a giant system of rings. When the companion was in the right place in its orbit, the rings passed in front of the star, blocking part of its light. Astronomers expected to see a similar dimming in 2017. They watched PDS 110 for weeks from a dozen observatories. But nothing happened. They also looked up observations made up to 50 years earlier — again with no big fades. Since the star is so young, it's surrounded by dust left over from its formation. Big clumps of dust might have passed in front of the star during the earlier events. But astronomers are still trying to figure out for sure what caused the big fades from this young star.  Script by Damond Benningfield Support McDonald Observatory

In the 1860s, a new meteor shower began to punctuate the December sky. And for more than a century afterwards, it got better and better. Now, it's become the most reliable shower of the year. The Geminid shower is at its best the next couple of nights. At its peak, it should produce a hundred or so meteors per hour. The Moon is in view for most of the night. But it sets early enough to provide a few good hours of meteor watching. Geminid meteors are bits of dust from an asteroid. The dust grains spread out along the asteroid's orbit. Earth intersects this path every December, so some of the grains ram into the upper atmosphere. They quickly vaporize, forming the glowing streaks of light known as meteors or shooting stars. The path didn't begin to intersect Earth's orbit until the 1800s, so the Geminid shower is the youngest of all the big meteor showers. The rate of visible meteors has increased since its discovery. That's because the gravity of Earth and Jupiter have shifted the path of the dust grains. Earth now plows more directly through that path — turning the Geminids into the year's best meteor shower. Look for the shower beginning in mid- to late evening. It usually produces a few especially bright meteors, so it's worth a look even through the moonlight. And although its peak is brief, it can produce the odd bright meteor for a night or two after the peak — extra time to catch the fireworks.  Script by Damond Benningfield Support McDonald Observatory

Summer is long gone, and winter's just more than a week away. Yet one of the signature star patterns of summer is still in great view in the evening sky. The Summer Triangle is well up in the west at nightfall. Its brightest point is the star Vega, about a third of the way up the sky. Deneb stands above it, with Altair well to the left of Vega. Deneb represents the tail of Cygnus, the swan. At this time of year, the swan is nose-diving toward the horizon. In this position, the star pattern looks a lot like a crucifix, so it's also known as the Northern Cross. It stands directly atop the horizon about 10 o'clock. Every star and constellation has its own season. That's because there's a difference in the time it takes the Sun and the other stars to return to the same position in the sky. The background stars — those we see at night — return to the same position every 23 hours and 56 minutes. But during that span, Earth moves a good distance in its orbit around the Sun. As a result, it takes four minutes longer for the Sun to return to the same position. The difference means that the distant stars all rise and set four minutes earlier each night, or two hours each month. The Summer Triangle first appears at nightfall around the start of summer. Over the following months it climbs high overhead, then drops down the western sky in autumn and into early winter — a remnant of the warmer months of summer.  Script by Damond Benningfield Support McDonald Observatory

To go to the stars, you need a starship. To power it, you need something other than rockets — like warp drive. And a recent study says building a warp drive might not be as tough as it first seemed. The closest star other than the Sun is Proxima Centauri — about four light-years away. Yet with conventional rockets, it would take 50,000 years to get there. A research project is trying to use laser-powered sails to cut that to just 20 years — but only for spacecraft weighing just a few grams. Warp drive would enfold a spacecraft inside a “bubble” of space-time — like being in its own universe. That bubble could travel at lightspeed or faster without violating the laws of physics. Early concepts, though, suggested it would take a non-existent form of energy to create the bubble. But a study by Erik Lentz, of a physics institute in Germany, suggested otherwise. Lentz did new calculations based on the theories of relativity. And he found that it could be possible to create a warp bubble with “normal” energy. And it could be done so that time would pass at the same rate for people inside the bubble as those on the outside. It would take ridiculous amounts of energy to create a bubble the size of a football field — smaller than any version of the starship Enterprise. But other studies have suggested ways to cut the energy needs — perhaps to a scale that could be powered by a nuclear reactor — carrying a starship at warp speeds.  Script by Damond Benningfield Support McDonald Observatory

If you're going to move to another star system, you might as well go big. That's the idea behind worldships — starships that would carry everything you'd need to set up a colony at another star. Worldships came from science fiction. But a few scientists and engineers have studied how they might work. They'd carry thousands of people — maybe tens of thousands. That means they'd be many miles long. And they'd carry everything needed to sustain the people on board for the trip. And it would be a long one. Many concepts of worldships envision peak speeds of maybe one percent of the speed of light. At that speed it would take more than 400 years to reach the closest star other than the Sun, Proxima Centauri. A ship would need to be shielded from radiation and collisions with space rocks, and it would have to be able to repair itself. The main challenge, though, might be social. The trip would last for generations — plenty of time for social order to break down. So the way the society is organized and controlled would need a lot of thought and T-L-C. A recent study said it could be centuries before the global economy could pay for a worldship. And it pointed out a possible problem that also shows up in science fiction: When a worldship reaches its destination, it might find the place already inhabited — by people traveling in faster starships invented long after the worldship left Earth. More about star travel tomorrow.  Script by Damond Benningfield Support McDonald Observatory

After slipping past Venus and Saturn the last couple of evenings, the Moon hangs out with one more planetary companion the next couple of nights: Jupiter, the giant of the solar system. The brilliant planet stands above the Moon as night falls tonight, and to the right of the Moon tomorrow night. Jupiter's most obvious feature is the Great Red Spot — a giant storm system. It towers high above the surrounding clouds, and it's been around for at least 150 years — and perhaps much longer. Compounds dredged up from deep below the storm color it in shades of red and orange. The Great Red Spot has received a lot of attention in recent years because it's been shrinking. A hundred and fifty years ago, it was between two and three times the diameter of Earth. Today, though, it's only about one-and-a-quarter times the size of Earth — about 10,000 miles across. A recent study found that the storm's winds are changing, too. Hubble Space Telescope keeps a regular eye on Jupiter. Scientists use it to track the winds across the planet — especially in the Great Red Spot. They found that, from 2009 to 2020, the wind at the rim of the storm sped up by about eight percent. It blows at a brisk 400 miles per hour — more than twice the speed of the most powerful hurricane ever recorded on Earth. On the other hand, winds at the center of the storm have slowed down — adding to the intrigue about this amazing storm.  Script by Damond Benningfield Support McDonald Observatory

The giant planets of the outer solar system have giant families of moons. Saturn, for example, has more than 80 known moons. The planets protect the moons by enfolding them in their own magnetic fields — some of the time. Saturn's magnetic field is generated deep within the planet. Saturn's core is made of rock and metal. It's surrounded by a thick layer of hydrogen. That layer is squeezed so tightly that it acts like a metal. The core and the metallic hydrogen rotate at different rates. That produces an electric dynamo that generates the magnetic field. Saturn's magnetic field is hundreds of times stronger than Earth's. It deflects much of the solar wind — a flow of charged particles from the Sun — as well as cosmic rays, which are particles from outside the solar system. But the solar wind sculpts the magnetic field into a teardrop shape. The region that faces the Sun is squeezed in toward Saturn. The region away from the Sun forms a tapered tail that extends millions of miles into space. So depending on how far it is from Saturn, a moon may be protected when it's on one side of the planet, but unprotected on the other — and subject to the full force of the solar wind. Look for Saturn near our own moon tonight. It looks like a bright star, just above the Moon at nightfall. They're flanked by two brighter planets — Venus to the lower right, and Jupiter to the upper left. More about the Moon and Jupiter tomorrow.  Script by Damond Benningfield Support McDonald Observatory

The Moon and the planet Venus put on a great show early tonight. Venus is the brilliant “evening star.” It stands just above the Moon. The Moon is a thin crescent — the Sun lights up only about one-tenth of the lunar hemisphere that faces our way. As the twilight begins to fade, you can make out the entire lunar disk. It's bathed in sunlight reflected from Earth. Since the invention of the telescope, many sharp-eyed observers have noticed a similar glow on Venus. It has nothing to do with Earth, though — the two planets are just too far apart. And it might not even be real. The glow on Venus is most often seen when the planet is an evening crescent, as it is now. In fact, it's never been recorded when the planet was more than about 30 percent “full.” It was first reported in 1643. It was called “ashen light” because it looks faint and grayish. And it's been observed many times since then. Scientists have proposed many explanations for the glow. Some have said it's produced by oxygen atoms in the upper atmosphere that are “energized” by the Sun. Others have suggested it's caused by big lightning storms. And still others say it's only an illusion; the glare of the bright crescent, combined with poor viewing conditions, can convince the eye it's seeing something that's not really there. If you have a small telescope, you can check it out for yourself — scanning the night on Venus for the possible glow of the ashen light.  Script by Damond Benningfield Support McDonald Observatory

The Sun isn't a very good clock. The time it takes to move from one “noon” to the next can vary by almost half a minute per day. And that creates some interesting effects. One example is that the year's earliest sunsets come days or weeks before the shortest day of the year. There are a couple of reasons for that. One is Earth's tilt on its axis. It causes the angle and location of sunrise along the horizon to change day by day. The other is Earth's lopsided orbit. That causes the Sun to return to the same point in the sky a little earlier or later in the day, depending on whether we're close to the Sun or far away. Earth is close to the Sun in December, so it moves faster than average. As Earth rotates, it catches up to the “noontime” Sun a little earlier each day—by as much as 30 seconds. Because of that, sunrise and sunset occur a little earlier, too. From here in the United States, the earliest sunsets of the year come a few days or weeks before the shortest day of the year — the winter solstice, on December 21st. The exact date of earliest sunset varies by latitude. From the southernmost latitudes, it happens in late November or early December. From the latitude of Denver or Philadelphia, it happens about December 10th. And from Minneapolis or Seattle, it comes closer to the solstice. The date of the latest sunrise comes about the same gap after the solstice — another result of the imperfect solar clock.  Script by Damond Benningfield Support McDonald Observatory

A star that was kicked out of its birthplace is really lighting things up. The supergiant star illuminates a cloud of gas and dust, creating a colorful display known as the Flaming Star Nebula. The nebula is in Auriga the charioteer, which is in the northeast at nightfall, marked by the bright star Capella. The nebula is to the lower right of Capella, and is an easy target for small telescopes. The star that's responsible for the nebula is AE Aurigae, and it's pretty amazing. It's more than 20 times as massive as the Sun, and about 60,000 times brighter. It probably was expelled from a stellar nursery in Orion as part of a ballet involving four stars. It got such a big “kick” that it's a runaway — it's moving far faster than most of the stars in the Milky Way. And right now, it's moving through a cloud of gas and dust that's several light-years across — the material that makes up the nebula. The surface of AE Aurigae is tens of thousands of degrees hotter than the Sun's. That's one of the keys to making the nebula shine, in shades of red and blue. Stars that are so hot produce a lot of ultraviolet light. It rips apart atoms of hydrogen. When the atoms come back together, they emit red light. And the blue light is a reflection — visible light from the star reflecting off small grains of carbon and other molecules. Combined, the red and blue make the nebula look like it's on fire — the Flaming Star Nebula.  Script by Damond Benningfield Support McDonald Observatory

A dead star in Aquila, the eagle, is a double record holder. It's the heaviest star of its type yet discovered — and also the smallest. It's only about 130 light-years away. But it's so tiny and faint that you need a big telescope to see it. The star is a white dwarf — the dead core of a once-normal star — or in this case, two stars. It probably formed when two white dwarfs merged. The combined star is 1.35 times the mass of the Sun. That's almost as heavy as a white dwarf can get. Yet it's only about as big as the Moon. The core of a star up to about eight times the mass of the Sun is kept “puffed up” by the radiation from its nuclear reactions. When they end, gravity wins out, and the core collapses to form a white dwarf. But it only collapses so far. That's because the charged particles known as electrons exert a pressure that keeps them apart. The heavier the white dwarf is, though, the tighter it's squeezed by gravity. Its electrons have to move faster and faster to limit the star's collapse. Eventually, though, the electrons just can't move any faster. They combine with protons to form neutrons. The neutrons are perfectly happy to jam together, so at about 1.44 times the mass of the Sun, a core collapses to form a neutron star — a body about the size of a small city. The star in Aquila isn't massive enough to become a neutron star. Instead, it'll remain a small but heavy white dwarf — the remnant of two dead stars.  Script by Damond Benningfield Support McDonald Observatory

In 30 million years or so, the Milky Way Galaxy may stage a spectacular fireworks display. It could give birth to hundreds of thousands of stars in just a few million years — the result of a massive collision. Smith's Cloud is falling toward the Milky Way's disk. The cloud is about 10,000 light-years long. To get a sense of how big that is, consider that it spans about 20 times the width of the full Moon as seen from Earth, even though it's maybe 40,000 light-years away. Astronomers are still trying to figure out where the cloud came from. A study a few years ago used Hubble Space Telescope to examine its chemistry. From that, scientists suggested that it came from the outer regions of the Milky Way's disk — perhaps blasted away by a cluster of exploding stars about 70 million years ago. Today, it's falling back toward the disk — toward the outer edge of the Perseus spiral arm. When it hits, it'll slam into other clouds of gas and dust. That will squeeze all the clouds, causing them to split into clumps. The clumps then will collapse to form new stars — the equivalent of perhaps a million stars the mass of the Sun. For now, Smith's Cloud is in the constellation Aquila, the eagle, which is low in the west-southwest at nightfall. Its brightest star, Altair, marks the left point of the bright Summer Triangle. Smith's Cloud is so cold and dark that it's visible only to radio telescopes — to the lower left of Altair.  Script by Damond Benningfield Support McDonald Observatory

The bright star Deneb stands high in the west-northwest at nightfall. It represents both the tail of Cygnus, the swan, and the top of the Northern Cross. And if you have a dark sky, you can see that it's immersed in the glow of the Milky Way — the light of millions of stars that outline the disk of our home galaxy. Many of those stars are in the same feature of the galaxy as the solar system — the Local Arm. It's one of several spiral arms that make the Milky Way look like a pinwheel spinning through space. For a long time, it wasn't considered an “arm” at all. Instead, it was classified as a “spur” — a short spike sticking off one of the major spiral arms. But over the last couple of years, several studies have found that it's a lot longer than thought. One study plotted the locations of hundreds of thousands of stars, plus hundreds of star clusters. The positions came from Gaia, a space telescope that's mapping more than a billion stars. Another study plotted the positions of “masers” — bright beams of microwaves energized by hot stars. Both studies found that the Local Arm extends for at least 25,000 light-years. That's not nearly long enough to loop all the way around the galaxy. And it may not “loop” at all — it may extend straight across the galaxy. Yet it's clear that the Local Arm is bigger than expected — an impressive neighborhood for the Sun, Deneb, and most of the stars we see in the night sky.  Script by Damond Benningfield Support McDonald Observatory

Nine billion years ago, another galaxy rammed into the Milky Way. It was only a tiny fraction the mass of the Milky Way, but it might have had a major impact on our home galaxy. Remnants of the galaxy were discovered within the last few years. It was given a couple of names: Gaia-Enceladus, and the Sausage, for the shape of its remains. According to simulations, the galaxy was only about one-and-a-half percent the mass of the Milky Way. As it plunged through the Milky Way's disk, though, its gravity stirred up the disk. That made the disk a lot “puffier” — it more than doubled in thickness. At the same time, clouds of gas from the interloper rammed into similar clouds in the Milky Way. The clouds collapsed to give birth to new stars — many millions of them. Those stars congregated along the plane of the disk, while many of the older stars stayed away from that plane. That gave the Milky Way two disks — a thin one within a thicker one — a structure it maintains even today. Finally, the gravity of the Milky Way ripped Gaia-Enceladus-Sausage apart. Many of the stars of the smaller galaxy formed long ribbons that loop around the center of the Milky Way. The stars belong to the Milky Way's halo — a thinly populated region that extends hundreds of thousands of light-years in every direction. In fact, they make up a large fraction of the halo's stars — stars that now belong to the Milky Way. More about the Milky Way tomorrow.  Script by Damond Benningfield Support McDonald Observatory

Our home galaxy, the Milky Way, is ancient. It was born perhaps 13.6 billion years ago — just a couple of hundred million years after the Big Bang. And it's been growing and changing ever since. The Milky Way doesn't have a birth certificate, so astronomers have to estimate its age. They do so mainly by measuring the ages of its stars. Stars of different ages have slightly different chemical compositions. The oldest stars have relatively small amounts of anything other than hydrogen and helium — elements created in the Big Bang. The amounts of those heavier elements act as a sort of clock, allowing astronomers to figure out when a star was born. Astronomers also use the “dead” stars known as white dwarfs — the bare cores of once-normal stars. A white dwarf is quite hot when it's born, but it cools off at a known rate. So measuring a white dwarf's temperature reveals how long it's been cooling off — and hence its age. Using these and other techniques, astronomers have found that the oldest stars in the Milky Way are more than 13 billion years old. But not all parts of the Milky Way are the same age; more about that tomorrow. In the meantime, if you have dark skies, look for the glowing outline of the Milky Way standing high in the sky this evening. After night falls, it arcs from Aquila, the eagle, in the west-southwest; to Cygnus, the swan, high in the west; to W-shaped Cassiopeia in the northeast.  Script by Damond Benningfield Support McDonald Observatory

The heart of the Sun is a toasty 27 million degrees Fahrenheit. At that temperature, atoms of hydrogen ram into each other and stick together to make helium. And they'll continue to do so for another five billion years. The cores of some stars are much hotter than the Sun's, so the nuclear reactions happen at a much faster rate. The key difference in stars is their mass. A heavier star has a more powerful gravitational pull. That squeezes its core more tightly. And the tighter the squeeze, the higher the temperature. An example is Spica, the brightest star of Virgo. It rises well below the Moon in the wee hours of tomorrow, but will be closer to the Moon the following couple of days. Spica consists of two giant stars. The heavier one — Spica A — is 11 or 12 times the mass of the Sun. With all those extra pounds, its core is tens of millions of degrees hotter than the Sun's. As a result, its fusion reactions take place far more quickly. So Spica A will use up its hydrogen in about 30 million years. After that, the core will shrink and get even hotter. That will trigger reactions with heavier elements. The star will step through that process several times, until its core consists of iron. At that point, despite a temperature of more than a billion degrees, the reactions will stop. Spica's core will collapse. Its outer layers will fall in, then rebound, creating a titanic explosion — bringing Spica A to a spectacular end.  Script by Damond Benningfield Support McDonald Observatory

Mars has many ways to “get” a visiting probe. It's cold, it's dry, and its air is thin, for example. But the biggest threat is dust storms. In fact, a giant storm may have doomed the first object to hit the Martian surface. Mars 2 was dispatched by the Soviet Union in May of 1971. It consisted of an orbiter and a lander. They arrived at Mars 50 years ago today. But a massive dust storm was under way. It covered the entire planet. Only the tops of a few tall mountains peeked above it. An American spacecraft, Mariner 9, had entered orbit around Mars a couple of weeks earlier. It was designed to be flexible. So when scientists saw that Mars was hidden from view, they waited a couple of months to begin its work. Mars 2 and a sister mission, Mars 3, didn't have that flexibility. They were programmed to dispatch their landers shortly before they arrived at Mars, and those instructions couldn't be changed. So the Mars 2 lander dropped into that massive dust storm on November 27th, 1971. And things didn't go well. The landing system malfunctioned, and the parachute didn't open — perhaps influenced by the dust storm. So the Mars 2 lander crashed — the first human artifact on the surface of Mars. The orbiter continued its mission, but it didn't see much through the dust. It did obtain important readings on the Martian atmosphere and magnetic and gravitational fields — adding to our then-meager knowledge of the Red Planet.  Script by Damond Benningfield Support McDonald Observatory